1. Field of the Invention
This invention relates to scanning probe microscopes (SPMs) and other related metrology apparatus. More particularly, it is directed to an apparatus and method for measuring the movement of a sample to be analyzed by an SPM, and to isolate its Z movement from parasitic X-Y movement generated by a Z actuator.
2. Discussion of the Prior Art
A scanning probe microscope operates by providing relative scanning movement between a measuring probe assembly having a sharp stylus and a sample surface while measuring one or more properties of the surface. The examples shows in FIGS. 1A and 1B are atomic force microscopes 10 and 11 respectively (“AFMs”) where a measuring probe assembly 12 includes a sharp tip or stylus 14 attached to a flexible cantilever 16. Commonly, an actuator such as a piezoelectric tube (often referred to herein as a “piezo tube”) is used to generate relative motion between the measuring probe 12 and the sample surface. A piezoelectric tube is a device that moves in one or more directions when voltages are applied to electrodes disposed inside and outside the tube (29 in FIG. 1C).
In FIG. 1A, measuring probe assembly 12 is attached to a piezoelectric tube actuator 18 so that the probe may be scanned over a sample 20 fixed to a support 22. FIG. 1B shows an alternative embodiment where the probe assembly 12 is held in place and the sample 20, which is coupled to a piezoelectric tube actuator 24, is scanned under it. In both AFM examples in FIGS. 1A and 1B, the deflection of the cantilever 16 is measured by reflecting a laser beam 26 off the back side 27 of cantilever 16 and towards a position sensitive detector 28.
One of the continuing concerns with these devices is how to improve their accuracy. Since these microscopes 10, 11 often measure surface characteristics on the order of Ångstroms, positioning the sample and probe with respect to each other is critical. Referring to FIG. 1C, as implemented in the arrangement of FIG. 1A, when an appropriate voltage (Vx or Vy) is applied to electrodes 29 disposed on the upper portion 30 of piezoelectric tube actuator 18, called an X and Y axis translating section or more commonly an “X-Y tube,” the upper portion may bend in two axes, the X and Y axes as shown. When a voltage (Vz) is applied across electrodes 29 in the lower portion 32 of tube 18, called a Z axis translating section or more commonly a “Z-tube,” the lower portion extends or retracts, generally vertically. In this manner, portions 30, 32 and the probe (or sample) can be steered left or right, forward or backward and up and down. This arrangement provides three degrees of freedom of motion. For the arrangement illustrated in FIG. 1A, with one end fixed to a microscope frame (for example, 34 in FIG. 1D), the free end of tube 18 can be moved in three orthogonal directions with relation to the sample 20. In addition with the X-Y tube 30 on top of the Z-tube 32 (i.e., furtherest from probe assembly 12), maximum X-Y range is realized.
Unfortunately, piezoelectric tubes and other types of actuators are imperfect. For example, the piezo tube often does not move only in the intended direction. FIG. 1D shows an undesirable, yet common, case where a piezo tube actuator 18 was commanded to move in the Z-direction (by the application of an appropriate voltage between the inner and outer electrodes, 29 in FIG. 1C), but where, in response, the Z tube 18 moves not only in the Z direction, but in the X and/or Y directions as well. This unwanted parasitic motion, shown in FIG. 1D as ΔX (not to scale), limits the accuracy of measurements obtained by scanning probe microscopes. Similar parasitic motion in the Y direction is also common. The amount of this parasitic motion varies with the geometry of the tube and with the uniformity of the tube material, but typically cannot be eliminated to the accuracy required by present instruments.
Current methods of monitoring the motion of the probe or sample 20 when driven by a piezoelectric tube in either the arrangement of FIG. 1A or FIG. 1B are not sufficiently developed to compensate for this parasitic X and Y error. The devices are typically calibrated by applying a voltage to the X-Y tube and the Z tube, and then measuring the actual distance that the sample or probe travels. Thus, the position of the piezo tube is estimated by the voltage that is applied to the X-Y tube and the Z tube. However, because the (X,Y) position error introduced by the Z tube on the probe (or on the sample for the arrangement shown in FIG. 1B) is essentially random, it cannot be eliminated merely by measuring the voltage applied to the Z tube or to the X-Y tube.
Moreover, with respect to movement in the intended direction, piezoelectric tubes and other types of actuators typically do not move in a predictable way when known voltages are applied. The ideal behavior would be that the actuator move in exact proportion to the voltage applied. Instead actuators, including piezo tubes, move in a non-linear manner, meaning that their sensitivity (e.g., nanometers of motion per applied voltage) can vary as the voltage increases. In addition, they suffer from hysteresis effects. Most generally, the response to an incremental voltage change will depend on the history of previous voltages applied to the actuator. This hysteresis effect, thus, can cause a large prior motion to affect the response of a commanded move, even many minutes later.
Additionally, vertical measurements in scanning probe microscopy are typically calculated mathematically by recording the voltage applied to the piezoelectric tube and then multiplying by the tube's calibrated sensitivity in nm/V. But as mentioned previously, this sensitivity is not constant and depends on the previous voltages applied to the tube. So using the voltage applied to the tube to calculate the vertical motion of the tube will always result in an error with respect to the actual motion. This error can translate directly into errors when measuring surface topography of a sample and performing other metrology experiments. These issues have been addressed specifically for the case in which the probe assembly of the AFM is coupled to the actuator (i.e., the case in which the probe assembly moves in three orthogonal directions, for example, in the cases cross-referenced above).
What is needed, therefore, is an apparatus and method for accurately measuring and controlling the motion of the sample or probe by minimizing adverse parasitic motion introduced by an actuator (e.g., a Z tube) in a metrology apparatus where the sample is scanned. In particular, if the adverse parasitic motion is minimized, the intended motion of the sample or probe will be realized and the apparatus will accurately measure and track the actual motion of the sample or probe in the X and/or Y directions in response to voltages applied to an XY actuator.